1. Field of the Invention
The invention relates to a method of camouflaging defective print elements in a printer having a printhead with multiple print elements, where each pixel of the image is assigned to a print element with which it is to be printed, and image information of a pixel assigned to a defective print element is shifted to a nearby pixel position where it can be printed by a non-defective print element. The invention also relates to a printer and to a computer program implementing this method.
2. Description of the Related Art
In an ink jet printer, the printhead contains multiple nozzles as print elements. Typically, the nozzles are arranged in a line that extends in parallel with a direction (subscanning direction) in which a recording medium, e.g., paper, is transported through the printer. The printhead scans the paper in a direction (main scanning direction) perpendicular to the subscanning direction. In a single-pass mode, a complete swath of the image is usually printed with a single pass of the printhead, and then the paper is transported by the width of the swath so as to print the next swath. That is, the single-pass mode is generally a mode where a complete line is printed by only one nozzle. When a nozzle of the printhead becomes defective, e.g. becomes clogged, the corresponding pixel line fails to print in the printed image, so that image information is lost and the quality of the print degrades.
A printer may also operate in a multi-pass mode, in which only part of the image information of a swath is printed in a first pass, and the missing pixels are filled-in during one or more subsequent passes of the printhead. In this case, a defective nozzle may possibly be backed-up by a non-defective nozzle, though mostly at the cost of productivity and added expense.
U.S. Pat. No. 6,215,557 typifies the related art discussed above, where a nozzle is defective, and the print data are altered so as to bypass the faulty nozzle. This means that a pixel that would have but cannot be printed with the defective nozzle is substituted by printing an extra pixel in one of the neighbouring lines that are printed with non-defective nozzles. The average optical density of the image area is therefore conserved, and the defect resulting from the nozzle failure is camouflaged and becomes almost imperceptible. This method involves an algorithm that operates on a bitmap that represents the print data, and shifts each pixel that cannot be printed to a neighbouring pixel position.
EP-A-0 999 516 discusses a method for generating a print mask that determines a pattern in which the pixels will be printed. This technology focuses on multi-pass printing, and the main purpose of the mask is to determine which pixels are to be printed in which pass. In the mask generation process, the print image information is only indirectly taken into account in the form of constraints that determine the construction of the mask. For example, such a constraint may require that a yellow pixel and a directly adjacent cyan pixel are not printed in the same pass of the printhead, in order to avoid color bleeding. This document further suggests constructing the masks in such a way that defective nozzles are backed up by non-defective nozzles.
These related art methods require that the locations of the defective nozzles are known before the respective image processing and mask generating steps can be performed. Thus, since the information on the locations of the defective nozzles originates from the printhead, either the printer itself must have sufficient processing capabilities to perform the camouflaging method, or the information on the locations of the defective nozzles must be reverse transmitted in the direction of the print data flow to a print driver or the like, where sufficient processing capability is available.